1. Technical Field
The invention relates generally to integrated circuit (IC) chip packaging, and more particularly, to an IC chip package having a force-adjustable member between the stiffener and printed circuit board (PCB).
2. Background Art
Referring to FIG. 1, conventional integrated circuit (IC) chip package 10 includes IC chip(s) 12 packaged on a single (not shown) or multiple chip carrier 14 that mates with a printed circuit board (PCB) 16 via a land grid array (LGA) connector 18. Chip package 10 further includes a module lid 26 and a metal stiffener 22, which may have a rigid insulator 24 therein to prevent metal stiffener 22 from shorting the underside of PCB 16. Module lid 26 may include a piston 28 and an adjustable spar 30, which provide thermal and mechanical accommodation of IC chip(s) 12. However, piston 28 and spar 30 are not always used. Metal stiffener 22 and module lid 26 are coupled together by screws/bolts 32 to hold chip package 10 together. A heat sink (not shown) may be provided over lid 26. LGA connector 18 may be interconnected to PCB 16 with a mechanical spring array (referred to as a double-LGA connector) or with a solder ball array (referred to as a hybrid LGA/BGA connector).
Ideally, chip package 10 has uniform force applied to chip carrier 14 and PCB 16. Although an individual spring within LGA connector 18 requires only 20-30 grams to insure robust performance over product lifetime, spring force is notoriously non-uniform, especially on a large multichip carrier 14. As a result, the spring force can vary at different locations on LGA connector 18. There are several causes of the non-uniformity of spring force. First, the retention screws/bolts 32 are of necessity outside the periphery of PCB 16. The collective spring forces thus tend to flex chip carrier 14, PCB 16 and/or metal stiffener 22, and locally tends to push parts away from each other. This situation also tends to relax contacts in the center of LGA connector 18. Second, PCBs 16 and chip carriers 14 originate having significant non-planarity, which varies in magnitude and pattern from part to part.
To insure that even the lowest force sites reach the minimum required spring force, the total clamping, or retention, force applied by screws/bolts 32 must be increased, typically to the equivalent of 90-105 grams per contact (i.e., a 4× overload). Unfortunately, the high clamping forces can introduce excessive flexural stress. One possible method to address this problem is to provide additional layers to the carrier, but this approach impacts cost and electrical performance. By another method, as shown in FIGS. 2 and 3, a polymer bulls-eye structure 36 may be added above rigid insulator 24 to help compensate for flexure by reinforcing the pressure at the center of chip package 10. Polymer bulls-eye structure 36 prevents having to increase the retention force of screws/bolts 32 even further, but requires dimensional consistency of the chip carrier 14, PBC 16 and stiffener 22. Unfortunately, the problems described above remain because the peripheral screws/bolts 32 are used both for chip package 10 assembly and to generate spring force.